Are specialized questions about some detail of a physics paper appropriate? (ie this)
I'm not sure what the community's thoughts on this are but I'd be inclined to say yes, in general such questions are appropriate. Certainly there is nothing inherently wrong with asking a very specific question. Of course it's possible that nobody on the site will know the answer, but that should not discourage you from asking.
Such a question could be inappropriate for other reasons, of course. For example, if your question is about pure math that just happens to be used in a physics paper, it may get closed as off-topic, because the mathematics SE site would be a better place for it.
I'm all in favour of this, since the main intention of physics.SE was to be a
Q&A site for active researchers, academics and students
This should include discussing details of publications, although I agree with David that pure math questions might be better off at math.SE.